Year	Percent
1960	33.878
1961	34.466
1962	35.06
1963	35.606
1964	36.122
1965	36.64
1966	37.162
1967	37.686
1968	38.215
1969	38.744
1970	39.278
1971	39.813
1972	40.352
1973	40.892
1974	41.457
1975	42.363
1976	43.276
1977	44.191
1978	45.111
1979	46.034
1980	46.961
1981	47.888
1982	48.818
1983	49.658
1984	50.438
1985	51.215
1986	51.993
1987	52.769
1988	53.546
1989	54.318
1990	55.09
1991	55.709
1992	56.226
1993	56.74
1994	57.254
1995	57.766
1996	58.277
1997	58.785
1998	59.291
1999	59.796
2000	60.299
2001	60.799
2002	61.118
2003	61.316
2004	61.514
2005	61.711
2006	61.907
2007	62.104
2008	62.3
2009	62.495
2010	62.69
2011	62.851
2012	62.988
2013	63.125
2014	63.261
2015	63.398
2016	63.534
2017	63.67